1. Field of the Invention
The present invention is related to voltage controlled oscillators and, more particularly to a quadrature voltage controlled oscillator formed on an integrated circuit chip.
2. Background Description
Voltage Controlled Oscillators (VCOs) are well known. A typical prior art VCO is a tunable tank circuit, i.e., an inductor (L) in parallel with a capacitor (C) driving a buffer, e.g., an inverter, that oscillates at a base frequency (T0) and has a voltage tunable operating range. Ideally, the VCO output frequency is directly and linearly proportional to a control voltage applied to the oscillator. Oscillator operating frequency may be varied by varying L or C. Generally, tunable tank circuit VCOs are the most reliable VCOs.
Completely integrated LC tank VCO""s typically require band switching techniques or even multiple VCO""s to achieve very wide tuning ranges. Discrete components, such as inductors and capacitors, are expensive and bulky. Further, attaching these discrete components to an integrated circuit complicates the integrated circuit. Accordingly, typical integrated circuit chip VCOs are based on simple oscillators or other circuits that may be built and contained on-chip, e.g., ring oscillators. However, ring oscillators are not particularly stable and are very sensitive to chip ambient and operating conditions.
Further, the output voltage swing of such ring oscillator based integrated VCOs is not constant but, is linearly related to output frequency. In order to achieve a large tuning range, the designer must accept small output signals at the low frequency end of the VCO operating spectrum. These low output signal levels degrade oscillator performance and increase noise sensitivity.
Thus, there is a need for a fully integrated quadrature VCO with a large tuning range and a large output signal level throughout the tuning range.